A photoacoustic apparatus used for medical diagnoses, for example, has been proposed as an apparatus that obtains information from an interior of a test subject (an object) using ultrasound waves (acoustic waves). The photoacoustic apparatus irradiates the object with laser pulsed light, and receives photoacoustic waves generated when tissue in the object absorbs energy from the irradiation light in a plurality of locations surrounding the object. Temporal variation in the received acoustic waves is then subjected to mathematical analysis processing, or in other words image reconstruction. As a result, information relating to optical characteristic values of the object interior can be visualized in two or three dimensions. Techniques of this type are also known as photoacoustic tomography (PAT).
An intensity of the acoustic waves generated from the internal tissue of the object varies according to an intensity of the irradiation light reaching the tissue and an absorption coefficient relative to a wavelength of the irradiation light. Many types of light-absorbing tissue, such as melanin, fat, water, hemoglobin, cholesterol, and collagen exist in the object, and each type of tissue has a different absorption coefficient relative to the wavelength. Therefore, when measurement is performed at different wavelengths and an image is reconstructed, different absorption coefficient distribution images are obtained even with respect to identical tissue in a single object.
A method of determining a blood oxygen saturation of the object using wavelength-dependent absorption coefficient distribution images is known. Two types of hemoglobin (oxyhemoglobin and deoxyhemoglobin) existing in blood vessels of the object have different absorption spectra, and therefore different absorption coefficient distribution images are obtained when measurement is performed at different wavelengths. A distribution ratio between oxyhemoglobin and deoxyhemoglobin (i.e. the oxygen saturation of the blood) can be calculated from these absorption coefficient distribution images. In NPL 1, for example, an oxygen saturation of a blood vessel is obtained from a reconstructed image using four wavelengths (578 nm to 598 nm).